We have previously isolated and characterized unique unesterified cholesterol (UC)-rich lipid particles that accumulate in atherosclerotic lesions of humans. Because human low density lipoprotein (LDL) is a cholesterol-rich lipid particle in the blood and has a molar proportion of cholesterol to phospholipid (3:1) similar to aortic UC-rich lipid particles, we investigated a possible precursor-product relationship between LDL and UC-rich lipid particles. We carried out enzymatic hydrolysis of LDL cholesteryl ester (CE) to determine if we could transform LDL to a particle similar to UC-rich lipid particles. No CE hydrolysis occurred when LDL was incubated with cholesterol esterase (CEase). However, when LDL was first incubated with trypsin and then incubated with CEase, LDL CE was hydrolyzed. This suggests that the protein moiety of LDL (apoB) acts as a barrier to block the susceptibility of CE to CEase. Trypsin pretreatment was not required for CE hydrolysis when LDL apoB was alternatively fragmentated by oxidation. When the same trypsin and CEase treatment was applied to high density lipoprotein (HDL), very little HDL CE hydrolyzed, suggesting that the structure and orientation of lipid within LDL and HDL result in a different susceptibility of their CE to hydrolysis. After complete hydrolysis of LDL CE, the degraded LDL particle transformed to uni- and multilamellar liposomal structures with sizes approximately 5 times larger than native LDL. The completely hydrolyzed LDL had structural and physical properties similar to those of UC-rich lipid particles that accumulate in human atherosclerotic lesions.